Turbine flowmeters have long been used to measure fluid flow for a wide variety of applications. One type of conventional flowmeter utilizes an axial-flow turbine disposed within a cylindrical bore of the flowmeter body. As fluid passes through the bore, it impinges upon the vanes of the turbine and causes rotation at a rate generally proportional to the rate of fluid flow. When the fluid is a liquid moving through the flowmeter at a moderate rate, the rate of rotation of the turbine is a fairly linear function of the fluid flow rate. Accordingly, some such conventional flowmeters, over a limited or narrow range such as one order of magnitude, exhibit one percent or better accuracy. However, when the flow rates are drastically reduced, the relationship between the rate of turbine rotation and the actual flow rate of the fluid is rather nonlinear so flowmeter accuracy is not maintained in such low-flow regions of operation.
Many electrohydraulic servo system applications, product packaging applications involving the dispensing of predetermined amounts of liquid such as the filling of beverage bottles, petroleum distribution applications such as retail sales of gasoline and scientific instrumentation applications would greatly benefit from turbine flowmeters which exhibit a high degree of accuracy over a broad fluid flow range of three or more orders of magnitude. To the best of our knowledge, conventional turbine flowmeters have not exhibited high accuracy over a flow range of two or more orders of magnitude. Normally, the problem of inaccuracy is most acute in the extremely low flow portions of the fluid flow range of the flowmeter.
In one model of prior art flowmeters, a magnetic pick-up coil through which current is run is used to detect rotation of the vanes of the turbine. The pick-up coil is placed within the sidewall of the flowmeter body, adjacent to the cylindrical bore in which the turbine is rotating. Only when the rotation rate is sufficiently high are the changes in the local reluctance adjacent the coil due to the vanes rapidly passing by, large enough to induce a detectable millivolt or microamp change in the current running through the coil. The frequency of the detectable change in current or voltage is directly proportional to the rotation rate of the turbine. Thus, a magnetic pick-up coil system inherently lacks the sensitivity required to produce a detectable electrical change when the turbine is rotating very slowly. So, in flowmeters using this type of vane sensor, the ability to accurately measure low flow rates is limited by the sensor itself as well as the nonlinearities previously described.
Another problem encountered with certain magnetic pick-up coil arrangements is that the magnetic field produced by the coil is strong enough to tend to stop a ferromagnetic turbine from rotating at extremely low flow rates. This occurs when the attraction of the ferromagnetic material in a vane by the magnetic field produced by the coil when the vane is close to the coil is stronger than the rotation-inducing mechanical forces of the flowing fluid. When this happens, the vane tends not to rotate freely away from the position of maximum magnetic coupling with the coil and introduces further nonlinearities at low flow rates. In one such flowmeter, we have observed a condition where the magnetic coupling was sufficiently strong to inhibit all rotation of the turbine below a flow rate of 0.5 gallons per minute (gpm) in a cylindrical bore approximately 0.7 (18 millimeters) in diameter, which thus effectively dictates the lower limit of operation of a turbine flowmeter.
We have found that certain magnetic proximity sensors which contain a tiny electronic circuit within the sensing head produce such tiny magnetic fields that they do no exhibit the foregoing magnetic coupling problem. The circuit within such magnetic proximity sensors includes an oscillator and amplifier so that an amplified pulsating analog output is produced directly from the sensor, thereby beneficially increasing the strength and sensitivity of the pulsating output signal.
While the later type of magnetic proximity sensors work well in low-temperature applications, they cannot be used in applications involving the measuring of higher temperature fluids. This is because the self-contained circuitry cannot withstand temperatures in excess of 65 degrees to 90 degrees Celsius (C.). Thus, such flowmeters cannot be used to measure the flow of a fluid whose temperature is within or in excess of these values without destroying the sensors, unless water cooling or the like is provided which, in many applications, is prohibitively expensive or impractical. Thus, temperature constraints imposed by existing magnetic proximity sensor technology is a real problem.
There are numerous applications for flowmeters in flammable or explosive atmospheres, such as are found in petroleum refineries, mills, mines, munitions plants, gasoline filling pumps and the like. In such applications, electrically-operated magnetic proximity sensors cannot be used unless they are enclosed within a suitable explosion-proof enclosure, which is, of course, very costly. We understand that there has been a longstanding need for a high-accuracy intrinsically safe turbine flowmeter which could be used in such hazardous environments.
Another problem with conventional electronic flowmeters using magnetic proximity sensors or magnetic pick-up coil circuits is that they also use analog electronic circuits which are rather inaccurate. Most such circuits take a pulsating analog signal and convert it into an analog DC voltage or current value, often by averaging several pulses together. The higher the resulting average DC current or voltage, the higher the flow rate is. Unfortunately, this can limit the overall accuracy and/or range of an electronic flowmeter. For example, if the analog current level is divisible into a maximum of 256 distinguishable levels, this limits over-all accuracy to about 1 percent for a flow range spanning 2.5 orders of magnitude.
There has been a longstanding need in the hydraulics industry for a way to test electrohydraulic servo valves to determine whether they are operating properly. Many of these servo valves have an extremely wide dynamic response range. To test such valves and to analyze the source of problems within such valves, it would be very useful to have a electronic flowmeter having an accuracy of greater than one percent over a range of flows three or more orders of magnitude. For servo valve analysis, it is particularly important to have accurate measurements at very low flow rates. We have been working on developing such a servo valve analyzer and have been unable to find such a highly accurate electronic turbine flowmeter on the market. Thus, it would be very desirable to have such a electronic flowmeter for the servo valve analyzer applications and for other instrumentation applications requiring high accuracy over wide dynamic range of flows and pressures.
In light of the foregoing problems and desires, it is a primary object of the present invention to provide an electronic flowmeter system which is capable of highly accurate flow measurements over a very wide range of flow rates, i.e., three or more order of magnitude.
It is another object of the present invention to provide a electronic turbine flowmeter system capable of making highly accurate flow measurements at low flow rates, that is, a flow rate wherein the turbine within the flowmeter rotates at a rate of less than five revolutions per second, and even at extremely low flow rates, that is a flow rate wherein the turbine rotates at a rate of less one revolution per second.
One more object of the present invention is to utilize digital electronics for substantially all of the measuring, timing and compensating steps within an electronic flowmeter for improved accuracy, thereby eliminating the limitations on accuracy or range imposed by using analog electronics and analog signal processing techniques.
Still another object of the present invention to provide an electronic flowmeter which utilizes an optical sensor capable of withstanding high temperatures and high pressures in the fluid whose flow is being measured.
It is another object of the present invention to provide a method of operating an electronic flowmeter so as to compensate for nonlinearities between the rate of turbine rotation and the rate of fluid flow under low fluid flow conditions.